Why, yes, I believe we do.
This is the way I, as the parent of an 'only', sees it: I *chose* to have one child. I chose this for a number of reasons (financial, repeated history of miscarriage, which ripped me open a number of times emotionally...)... the choice to have one child is to benefit our whole family and keep it functioning well. I also nannied for several families with multiples. I knew what I was choosing.
That said, people who have multiples also *chose* to have multiples. If someone were to tell me I had 'no right' to complain, I might remind them that it was *their* decision to have more children, not my decision for them. If we are all being honest and responsible for ourselves, then this is a no-brainer.
There are always trade-offs to having singles or multiples. Multiples have horrible sibling arguments to deal with. I enjoy not having to deal with that sort of problem-solving regularly. (I did it all day when I taught preschool.) That said, siblings have someone to go play with... my son does not and consequently, he needs me more intensely than kids with sibs might need their parents. (notice I say "might", every kid is different.) I spend a LOT of time with my son. I don't enjoy every minute of it, but in accepting responsibility for our decision to have just one: this IS what I signed up for. My son doesn't have a built-in playmate, so I do have to give him a lot of my attention.
To me-- and I could be totally wrong-- if we are all taking care of ourselves and our own stuff, being real with life-- I don't see how my 'complaining' from time to time should matter to or take away from another person who chose to have more children than I did. That's a bit like saying that stay-at-home parents shouldn't complain because they don't 'work as hard' as parents who work outside the home, or that married parents don't have a right to gripe because hey, look at how hard the single parents has to work.
Life is full of choices. Choices to reproduce with someone you want to spend your life with-- or not. Choices to use contraception or wing it. Choices to plan to have more babies (even to the extent of medical intervention) or to be happy with what one has, here and now. I strongly believe that these are all choices, and each choice has its consequences. Whether they are favorable or hard-- that too is a matter of perception.
I have a neighbor with three children, a dizzying schedule and constant chaos in her life. Guess what? It would drive me nuts, but she thrives on this. So again, all a matter of perception and personal preference.
As long as we believe we are not victims, but are indeed responsible for our own choices and the authors of our own lives, someone else's complaining is NOT going to take away from who we are or what we do.
(I am aware that some parents deal with the death of a spouse or natural disasters or debilitating medical issues...those are NOT choices, and so I am not factoring those circumstances into this question.)
PS: if you read through A.'s profile questions, you'll see that she too has one child. I don't believe she's feeling entitled or trying to stir the pot by posting this question. It's actually an interesting question...